The question ‘Does God exist?’ seems to have been present in every culture and philosopher.
There is one philosopher however who can be singled out as the thinker whose remarks on the existence of God have been quoted the most: Saint Thomas Aquinas (1225-1274.)
Even contemporary crawler-based search engines corroborate this fact.
Therefore, any serious attempt at answering the question ‘Does God exist?’ would be incomplete if it failed to mention Aquinas’ treatment of this issue.
In this article, I provide an answer to the question ‘Does God exist?’ by examining the most distinctive characteristic of Aquinas’ contribution to the subject.
It is generally accepted that Aquinas’ Five Ways of proving the existence of God have their ultimate source in Aristotle. Aquinas is not the originator of the arguments.
But did Aquinas say anything novel concerning the existence of God? The answer is a definite yes.
Aquinas offers another argument in his philosophical and theological writings to answer the question ‘Does God exist?’ It is the argument based on the notion of the ‘actus essendi.’
The discovery of the notion of ‘actus essendi’ equipped Aquinas with the metaphysical principle he needed to formulate an original and incisive argument for the existence of God. The ‘Actus Essendi Way’ developed by Aquinas sets him apart from all other philosophers, including Aristotle.
As Pope John Paul II (1920-2005) remarked,  the transcendental value of the ‘actus essendi’ ‘paves the most direct way to rise to the knowledge of subsisting Being and pure Act, namely to God.’
Here is a brief explanation of the term.
The expression ‘actus essendi’ is a technical term used by Aquinas in its restricted meaning. ‘Actus essendi’ is the metaphysical principle that goes ‘side by side’ with the metaphysical principle ‘essence’ in a subsistent extramental thing.
Three points of reference are indicated here. One, the real finite thing itself existing in the external world; another, the ‘essence’ which makes the thing to be what it is; and yet another, the ‘actus essendi’ which places both the thing with its ‘essence’ in actual existence.
In the real world ‘essence’ and ‘actus essendi’ are inseparable metaphysical principles. The metaphysical principle of ‘actus essendi’ always appears instantiated in an ‘essence.’ And the ‘essence’ of the thing is what put limits to the thing’s participation in ‘actus essendi.’
Now, returning to the question ‘Does God exist?’ the argument for the ascent to God based on the metaphysical notion of ‘actus essendi’ is not an ‘a priori’ argument. It is an inductive argument.
The initial observation is this: In every existing thing, that which is unique and most fully singular in it is its ‘actus essendi.’
Here is the argument in brief.
Everything that pertains to a finite thing is either caused by the principles of its own ‘essence,’ or by some extrinsic principle.
Now, it cannot be that the ‘actus essendi’ of a finite thing is caused by the ‘essence’ of the thing as if the ‘essence’ were an efficient cause, because then the thing would be the efficient cause of its own existence, and the thing would produce itself, which is impossible. The thing would be an entity able to act previous to its own existence, which is a contradiction.
Therefore, whatever must have existence added to its ‘essence’ in order for it to be real, must be caused by another. Finite things, in which the metaphysical principle of ‘actus essendi’ is other than the metaphysical principle of ‘essence’, have their ‘actus essendi’ caused by some extrinsic principle.
Now, all that which is by participation leads back to that which is by ‘essence’ as to the first and highest. And since all existing finite things participate in ‘actus essendi,’ there must be, at the summit of all things, something in which the ‘essence’ does not limit the fullness of the ‘actus essendi.’
If this were not the case, we would have to go into an infinite regress in causes, for everything that is not the fullness of ‘actus essendi’ has to have a cause of its ‘actus essendi.’
Thus, because everything that is through another is reduced to that which is through itself as to a first cause, there has to be one extrinsic principle that is the cause of the ‘actus essendi’ in all things. And this is God, who is the most sufficient, the most worthy and the most perfect cause of all being, from whom all that which is, participates in ‘actus essendi.’ In God, and only in Him, ‘essence’ puts no limit to the ‘actus essendi.’
If God were not His own ‘actus essendi,’ He would be being by participation and not being by ‘essence.’ He will not, therefore, be the first being – which, as shown, would lead to an absurd infinite regress.
God is the ‘Ipse Actus Essendi subsistens
Therefore, there has to be someone who is the ‘first’ cause of the ‘actus essendi’ of all finite things. And this is God. God is the ‘Ipse Actus Essendi subsistens,’ subsisting act of being. God exists and is the source of all being.
With the metaphysical principle of ‘actus essendi’ as the underlying support for the major premise, the certitude of the reasoning towards the conclusion ‘Deus est,’ ‘God exists,’ was elevated to a considerably higher level.
Aquinas’ discovery of the notion of ‘actus essendi’ generated what is widely considered the finest answer was ever given to the question ‘Does God exist?’